chore: add script to generate twenty-ui barrels before build (#4707)

Split from https://github.com/twentyhq/twenty/pull/4518

Part of #4766 

Adds a script to auto-generate twenty-ui exports in `index.ts` files.

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Co-authored-by: Lucas Bordeau <bordeau.lucas@gmail.com>

packages/twenty-ui/src/index.ts

@@ -1,4 +1,5 @@
 export * from './components';
 export * from './display';
+export * from './testing';
 export * from './theme';
 export * from './utilities';